| Pipeline is the core component of the main circuit system in pressurized-water reactor.Because the working environment is characterized by high temperature,high pressure,direct contact with corrosive medium,large temperature gradient and radiation damage,higher requirements are put forward for the material properties of the pipeline.TP321 stainless steel pipe has become a common material for nuclear power plant pipelines because of its excellent high temperature mechanical properties,corrosion resistance,cold and hot working properties.Based on the production of TP321 seamless tube by domestic stainless steel production enterprises in the process of "arc furnace→AOD→argon blowing station→mold casting→stainless steel tube blank",the unqualified ultrasonic inspection of seamless steel tube frequently appeared in the production are deeply studied in this paper.The cause and formation mechanism of unqualified inspection are found out,and the improvement and control technology are put forward.By dissecting the sample of TP321 stainless steel pipe which failed ultrasonic inspection,it is found that the large size inclusions are the main reason leading to unqualified inspection.The formation mechanism of large size chain TiN inclusions and complex large size oxides containing SiO2 was identified through sampling,laboratory exploration and thermodynamic calculation.On this basis,through AOD refining model for oxidation stage and thermodynamic equilibrium model between slag and steel for reduction and Ti alloying process,the influence of refining process on the formation of Ti containing inclusions was investigated,and the relevant control technology was proposed.The following research results were obtained:(1)The ultrasonic inspection defects of TP321 stainless steel pipe are caused by the chain inclusions with a large size of 1～5mm and the layered inclusions with a length and width larger than 1cm,which are mainly located in the 1/3 wall thickness area from the inner surface of the steel pipe.The large size chain inclusions are mainly aggregated TiN inclusions,which can be divided into single TiN inclusions cluster and complex cluster composed of MgO-Al2O3-TiOx oxide and TiN inclusions.The size of complex TiN inclusion cluster is obviously larger than that of single TiN inclusion cluster.The large-size layered inclusion is a complex large-size oxide containing SiO2,which is composed of 40%SiO2-15%MnO-15%Al2O3-15%CaO-TiOx-Cr2O3 composite oxide with low melting point and coated granular TiOx inclusion.(2)The solidification experiment of 1.4kg small ingot in the laboratory showed that the size and quantity of TiN were closely related to the solidification cooling rate.From the ingot surface to the center,TiN particles gradually grow up and their number gradually decreases.At the central location,the TiN size reaches a maximum of 8μm,while the quantity is reduced to 108 pieces/mm2.Further dissections of the 1.8t and 7t stainless steel tube balnk from practice also show that the initial solidification zone of the outer layer of the tube balnk shows similar rules to the above laboratory results.However,with the solidification process,the segregation of Ti and N makes TiN particles continuously precipitate and grow,or even merge.Therefore,TiN quantity shows a lowest point,which is located at 1/2 radius from the center of the head end of 1.8t stainless steel tube blank and 3/4 radius from the center of the head end of 7t stainless steel tube balnk.Theoretical models verify and explain the above rules.The larger the ingot shape,the larger the TiN particle size and quantity in the final solidification area of the ingot,and it is easy to form chain TiN inclusions.(3)Formation mechanism of complex large-size inclusions containing SiO2:In reduction period of AOD furnace,when the content of oxygen in liquid steel is more than 300ppm,SiO2-MnO-Al2O3-Cr2O3 composite oxide is generated.Then it is modified by Ca and Ti in liquid steel to 40%SiO2-15%MnO-15%Al2O3-15%CaO-TiOx-Cr2O3 composite oxide with low melting point;In the Ti alloying process,TiOx inclusions are formed when the content of oxygen element in liquid steel is more than 100ppm.Due to strong stirring by side-blowing argon of AOD furnace,the above composite oxides with low melting point and high SiO2 collide with TiOx inclusions,forming complex and large size inclusions containing SiO2,and then forming large size layered inclusions in the prodcution of seamless tube.(4)After Ti alloying,the typical inclusion in the molten steel is MgO-Al2O3-TiOx oxide with different TiOx content.With the different content of TiOx,the properties of the compound TiN inclusions formed by the oxide and TiN are obviously different.When the content of TiOx is higher than 50%,the melting point of such inclusions is low,and the inclusions are not easy to aggregate and distribute uniformly.When the content of TiOx is less than 50%,the melting point of such inclusions is high and they tend to aggregate.The latter tends to promote the formation of large size chain TiN inclusions.The different TiOx content is mainly related to the degree of Ti modified MgO-Al2O3 spinel.The results show that the content of TiOx in MgO-Al2O3-TiOx oxide should be controlled to be higher than 50%,the content of Al2O3 to be lower than 25%and the content of MgO to be lower than 25%.(5)According to the AOD refining model for oxidation stage,the optimization technology of oxidation stage is proposed:the temperature of molten steel in AOD furnace is increased to 1550℃;When the temperature in AOD furnace reaches about 1700℃,lime,alloy and other cold materials are added in batches;The blowing system in oxidation period of 20t AOD furnace is divided into three stages,namely Ⅰ(oxygen flow=1200Nm31h,argon flow=240Nm3/h),Ⅱ(oxygen flow=600Nm3/h,argon flow=600Nm3/h),Ⅲ(oxygen flow=360Nm3/h,argon flow=720Nm3/h).After the furnace temperature control is stabilized,it can avoid the heating through oxygen blowing in the late oxidation period,reduce the loss of chromium element,avoid the peroxidation of liquid steel,and prevent the formation of layered inclusions with large size in the steel pipe.(6)Through the study of thermodynamic equilibrium model between slag and steel for reduction and Ti alloying process in AOD furnace,it is proposed that the composition of liquid steel should be controlled to be 0.4%Si and 0.003%A1 during reduction.The slag composition is controlled 50%CaO,30%SiO2,8%Al2O3,10%MgO,so that the Cr2O3content in slag is less than 1%.In the process of Ti alloying,when the mass fraction of Ti in molten steel is about 0.5%,the content of Al element is controlled to be about 0.02%.The slag composition is controlled to be 45～55%CaO,5～10%MgO and 5～10%CaF2,so as to ensure the yield of Ti alloy,reduce the generation of(MgO-Al2O3)rich-TiOx with high melting point and large size,and prevent the appearance of large size chain TiN inclusions in the steel pipe. |
PDF Full Text Request